Sound reproducer



April 12, 1932. F. A. MITCHELL SOUND REPRODUCER Filed Sept. 20, 192 2 Sheets-Sheet l FREQUENCY INVENTOR. l7" 6' Ffarzk fllZe/z Mza/ieil ATTORNEY.

April 12,1932. F. A. MITCHELL 1,853,167

SOUND REPRODUCER Filed Sept. 20, 1926 2 Sheets-Sheet 2 I A; a

' llllllllill J1 mil fl INVENTOR. F k ll 22% Zak ll fl la f e Patented Apr. 12, 1932 UNITED STATES PATENT OFFICE FRANK ALLEN MITCHELL, OF LONDON, ENGLAND, ASSIGNOR TO COLUMBIA PHONO- GRAPHCOMPANY, INC., 013 BRIDGEPORT, CONNECTICUT, A CORPORATION OF NEW YORK SOUND REPRODUCER Application filed September 20, 1926, Serial No. 136,415, and in Great Britain October 27, 1925.

has hitherto been obtainable with non-amplifying sound reproducers.

. My invention consists in a reproducer for sound-reproducing mechanism comprising a stylus bar, a stylus-operated diaphragm or equivalent vibrating member and a supporting frame for the vibrating member, characterized in that the said frame is pivoted to a reproducer-supporting member on an axis permanently situated between the axis of the said stylus-bar pivot and the point of maximum wave generation between the said vibrating member and the said reproducer-supporting member.

The invention also consists in a reproducer for sound-reproducing mechanism in which the sound-box or electric transmitter (hereinafter called reproducer) is mounted in relation to the tone-arm or other reproducercarrying member (hereinafter called tonearm) in such a manner that when the diaphragm or diaphragm equivalent (hereinafter called diaphragm) has moved to its elastic limit under the action of the record groove upon the stylus-bar, any further movement that may be given by the stylus bar impulse is allowed for by movement of the diaphragm-supporting case.

A further feature of the invention lies in mounting the diaphragm-supporting memher or case of the reproducer so that it is positively pivoted or hinged to the tone-arm about i an axis substantially parallel to the axis of the stylus-bar pivot, the diaphragmsupporting member or case being preferably maintained in a relatively constant but not rigid position upon the tone-arm by means of one or more springs or suitable fastenings.

One advantage of mounting the reproducer in the above manner is that the low frequen- .cy vibrations of the stylus bar are transferred to the diaphragm or like vibrating member .and are not passed on to the tone-arm (where they are ineffective) asis usual with noncushion-mounted reproducers, and the invention is to be distinguished from those reproducers which are cushion-mounted by the aid of rubber or by a suitable spring by reason of the fact that these latter do not limit the movement of the reproducer in relation to the tone-arm to one defined direction, Whereas in the case of reproducers mounted in accordance with the invention the reproducer is only capable of movement in one defined direction around a fixed pivot or axis, as is clearly set forth in the accompanying drawings. in which Fig. 1 is a section through a mechanical or acoustic reproducer constructed according to this invention. 8

Fig. 2 is a back view of Fig. 1 showing a method of hinging.

Figs. 3, 4t and 5 are diagrammatic sketches showing the purpose and application of the inventlon.

Fig. 6 is a graph showing the response advantage obtained from a reproducer constructed according to the invention.

Fig. 7 is a sectional view on the line 7-7 of Fig. 8 showing another convenient form of pivoting.

Fig. 8 is a rear elevation of the reproducer shown in Fig. 7.

Fig. 9 shows an electric reproducer using a metal diaphragm.

Figs. 10 and 11 show an electric reproducer using a moving armature in place of the usual diaphragm.

Figs. 1 and 2 show one convenient construction in accordance with the invention.

The diaphragm 15 of the usual form is mounted between rubber gaskets 16 in a reproducer case or frame 17. A stylus bar and needle holder 18 is flexibly pivoted at 19 to the reproducer rim in the usual manner. Springs 19a maintain the stylus bar on its pivots, and keep it normally central.

The back of the case 17 is provided with two lugs 20 having pivot pins 21 or balls mounted therein on a line approximately parallel. to the stylus-bar pivot 19, while the tone-arm connector 22 (which is rigidly secured to the usual tone-arm of a phonograph) is provided with recesses in which the pivot pins 21 engage, it being understood, however, that the pivot pins or balls may be carried upon the tone-arm end while the recesses are upon the back of the reproducer case or rim.

By the means so described the reproducer case 17 is hinged so as to be movable to and from the tone-arm connector 22, but is rigidly held against any other relative turning movement.

The free movable part of the case 17 is flexibly tied to the connector 22 by means of the screw or pin 23 which engages small compression springs 24 and 25, one on each side of the flange of the connector 22.

These springs 24 and 25 serve to maintain the reproducer case 17 in arelatively constant but not rigid position upon the connector 22. The relative flexibility between the case 17 and the connector 22 may be adjusted by turning the screw 23 or by other suitable means.

A rubber or other sound-tight cushion 26 is provided between the back of the reproducer 17 and the tone-arm connector 22.

The underlying principle of the invention will now be described with reference to Figs. 3, 4 and 5 which show a diagrammatic arrangement of a reproducer constructed as above described with reference to Figs. 1 and 2.

Fig. Sshows the repro'ducer parts in their normal quiescent positions, the recovery springs being omitted for clearness.

lVhen the record groove (while playing a record) moves the needle 27 in the direction of the arrow, Fig. i, it causes the stylus bar 18 to rotate on its axis 19 and push the diaphragm 15 inwardly, as shown by the arrow, and thus cause an increase in the pressure sound wave at the point of maximum wave generation 28 between the vibrating member 15 and the reproducer-supporting member 22.

It Will be understood that the greatest movement of the diaphragm will take place at the point where the stylus arm 18 is connected with the diaphragm. That point is the most remote from the clamping or mounting rings 16 and therefore has the greatest freedom of movement. It is this point, the point of the diaphragm or vibrating member which has the greatest movement that is called the point of maximum wave generation.

High audio frequency vibrations applied 1 to the needle point 27 will act to move the diaphragm 15 in and out without moving the case 17 owing to the inertia of the much heavier case 17 being too great to permit of its being eifectively moved above a certain (low) frequency.

When the needle 27 is moved through a greater distance by low frequency sound waves being generated by the record groove (note :the amplitude of a sound wave increases inversely as its frequency for a constant intensity of sound) the diaphragm is moved to its elastic limit and the stylus bar then functions to move the pivot 19 in the direction of the arrow; this causes the case 17 to rotate around the axis 21 so that the top also approaches the support 22 and thus adds its movement to the diaphragm 15 to increase still further the sound wave pressure at the point 28.

When the record groove moves the needle 27 in the opposite direction, as shown in Fig. 5, the stylus bar first functions to pull the diaphragm 15 outwards and any further low frequency movement of the needle 27 then reacts on the pivot 19 so as also to move the case 17 outwards as shown. This produces the con'iplementary (rarefied) sound wave (at the point of maximum wave generation 28) to that shown in Fig. 4-.

The eifect of this additional rcproducer diaphragm response to low frequency vibrations is shown in the graph, Fig. (3, in which the usual reproducer diaphragm response to various audio frequencies is indi cated by the line 29. This represents a diaphragm having a natural frequency 30 of about 1000 per second, which is usual for an ordinary mica diaphragm as commonly used.

The line 31 indicates the additional response obtained by the movement of the diaphragmsupporting case when mounted according to my invention, which additional response cannot be obtained by other known types of reproducer mounting.

The falling off or loss in response to very low frequencies as denoted by 32 (Fig. 6) is due to movement of the tone-arm and connector 22 which must necessarily be mounted on ordinary phonographs so that they can move freely across the record while playing. If the tone-arm connector had great mass and could not swing to and fro fairly easily there would be very little loss in these very low frequencies below It is obvious that the resistance of the tonearm can be adjusted to give the most suitable acoustic results consistent with reasonable wear on the records.

The combined response to acoustic frequencies of both the diaphragm 15 proper and the diaphragnii-supporting case 17 is in-' dicated by the line 33. It will be seen that the response efficiency is much greater on all the lower frequencies (below 00 for example) than is the case with a diaphragm mounted in any well-known manner, the net result being analmost equal value in response between about 32 vibrations per second and 512 vibrations per second;

Another useful form of pivoting between the case 17 and the connector 22 is shown in Figs. 7 and 8. In principle the action is the same as in Figs. 1 and 2 with the added advantage that the pivot 21 will not wear loose and thus lose in efiiciency. The total movement of a diaphragm when playing a record varies between .001 and .008 and it will be seen that any looseness at the pivots can easily absorb this movement before any useful work has been done.

' The tone-arm connector 22 is provided with two hard steel pivot points 21 7) which engage in two hard steel conical recesses 34 mounted in the case 17. The points 21 are held in their recesses 34 by means of a strong spring and screw 35 mounted in a simiiar way to the spring 24 and screw 23 seen in Figs. 1 and 2.

A projecting sleeve 36 enters the connector 22 and functions to prevent air leakage at the flexible joint. The stylus bar 18 is mounted between pivot pins 19 to minimize any vibration due to the elasticity of the pivot mount- T ig. 9 shows one form of electric reproducer so constructed that vibrations of the diaphragm or vibrating member 37 are transformed into electrical energy which may be amplified in any well-known way by means of thermionic valves or otherwise.

The diaphragm is conveniently constructed of soft iron mounted between gaskets and connected to the stylus bar 18 as before c escribed although in this form of reproducer it is not essential to use gaskets as air leakage is of little importance.

The tone-arm connector 22 carries a magnet 38 on which coils 39 are mounted in the manner usual with telephones. A protective covering 40 formed as part of the connector 22 is provided with a projection to which is attached a spring 41 so arranged that it acts as a pivot between the connector 22 and the case 17 to which it is also attached at 42. This spring 41 also serves to keep the parts in a relatively constant position, it being unnecessary to use an accepted form of pivot or hinge 21 as the movements are so microscopic as before explained.

Any movement of the needle 27 when reproducing a record is transmitted to the vibrating member 37 either direct or by way of the case as before described and acts on the magnet 38 at the point of maximum wave generation 28 between the vibrating member 37 and the reproducer-supporting member 22. This generates a varying electric current in the magnet coils 39 which are connected through the wires 43 to any suitable amplifier.

Figs. 10 and 11 show the principle of the invention applied to a more efficient form of electric reproducer in which a moving soft iron armature 44 is used in place of the usual form of diaphragm as a vibrating member.

This vibrating member 44 is pivoted at 5-0 in a case or frame 45 which is in turn pivoted at 21 to the walls 46 of the connector 22 which is connected to the tone-arm or tone-arm equivalent in the usual way. As in Fig. 9, the magnet 38 and the coil 39 are carried by the connector 22, an additional magnetizing coil 47 being provided to keep the magnets up to strength. The magnet poles on one side 48 are north for example and those on the other side 49 are south.

Vibrations applied to the needle 27 will move the stylus bar and vibrating member 44 about the axis 50 and will cause the member 44 to change its magnetic polarity according to which of the magnet poles 38 is nearest thereto. If the member 44 is near the north pole 48 and the bottom end 51 is near the south pole 49 the vibrating (soft iron) member 44 will become magnetized so that its south pole is at the top and if the member 44 is moved by the needle 27 so that its ends are near the opposite poles the magnetism of the member 44 will be reversed and this generates a current in the coil 39 which is connected by the wires 43 to an electric amplifier as before described. p

Now if the movements of the needle 27 are email and of a high audio frequency, only the member 44 wiil move, it being kept normally central by means of the springs 52 which function in a similar manner to 19a in Fig. 1, but should the vibrations be greater and of a lower frequency the frame 45 will also move (around the pivots 21) owing to its inertia being overcome as before described. This will allow the vibrating member 44 to approach nearer the magnet poles 48 and 49 and thus generate a greater current in the coil 39. The frame 45 is maintained in a relatively constant position by means of the screws 23 and springs 24. It will be seen that the point 28 of maximum wave generation between the vibrating member 44 and the connector 22 is between the poles of the magnets 38, this being the portion of the member most remote from its pivot or anchorage and the portion capable of the great est amount of movement.

' It should be noted that in all the above examples the proportion and direction of relative movement between the diaphragm or equivalent vibrating member 15 and the case are determined by the inertia of the parts and the position of the pivots or hinge between the back of the reproducer case 17 and the tone-arm equivalent 22. The exact weigl'its elasticities and pivotal positions are determined by the proportion of response desired between the lower frequencies and the upper frequencies but it is considered advisable to locate the pivot 21 on an axis situated between the stylus bar axis 19 and the point 28 of maximum wave generation between the vibrating member 15 and the reproducer-supporting member 22.

It should be understood that the principle hereinbefore described is applicable to almost any type of reproducer either of the electro-magnetic, condenser or resistance (carbon granule) electric type or the usual diaphragm type of acoustic reproducer, and

with said diaphragm, a supporting frame for the diaphragm, a reproducer-supporting member, means pivoting the frame to said member on an axis permanently situated between the axis of the said stylus bar and the point of connection of the stylus bar with said diaphragm, the pivot of said frame be ing on an axis substantially parallel to the axis about which the stylus bar moves, and means holding said frame in a relatively constant but not rigid position relative to the reproducer-supporting member.

2. In combination in a reproducer, a supporting frame, a stylus operated vibrating member in said frame, a stylus mounted for movement about an axis and connected with said vibrating member, a reproducersupporting member, the point of maximum wave generation between. the said vibrating member and the said reproducer-supporting member being adjacent the upper edge of said vibrating member, means pivoting the supporting frame to the reproducer-supporting member on an axis permanently situated between the axis of the stylus bar the point of maximum wave generation between the said vibrating member and the said reproducer-supporting member.

3. A reproducer as in claim 2 wherein the frame for the vibrating member is pivoted to the reprodnoer-supporting member on an axis substantially parallel to the axis about which the stylus bar moves.

4. A reproducer as claimed in claim 2 including means for holding the supporting frame in a relatively constant but not rigid position relative to the reproducer supporting member. 7

5. In combination, a reproducer, a connector, means for pivotally securing the reproducer to the connector at one side of the center of the reproducer, means connecting the reproducer to the connector at the opposite side of the center of the reproducer, said last named means including means for yieldingly resisting movement of the reproducer on said pivot in directions toward and from the connector.

6. In combination, a frame, a stylus carrier pivotally mounted by the frame, and means pivotally mounting the frame for movement in one plane and rigidly against movement in any other plane, and on an axis substantially parallel to the axis about which the stylus carrier moves, whereby excessive movement of the stylus carrier results in movement of the frame about its pivot.

7. In a reproducer, a reproducer support, a vibrating member, a frame for said vibrating member and supporting the same, a pivot connecting said frame to said reproducer su port for movement in one plane but rigid y holding it against movement in any other plane, a pivoted stylus bar adapted to be moved by contact of a stylus with a record groove and rigidly connected to and adapted to move said vibrating member, said pivotal connection for the frame providing means whereby when the vibrating member has been moved to its elastic limit by the stylus bar, further movement of said vibrating member is effected by movement of the frame about its pivot.

8. A reproducer as claimed in claim 7 and including means connecting the frame and support and holding said frame in a relativel v constant but not rigid position relative to the support.

9. In combination, a reproducer including a frame and a stylus driven vibrating member, a stylus holder connected with the vibrating member, a support for connecting said. reproducer to a tone arm, means pivotally securing the reproducer frame to the support at one side of the center of the support, means securing the reproducer frame to the support at the opposite side of the center of the support, said last means including spring means serving to yieldingly retain the frame in a neutral position on its pivot and against movement by the vibrating member under the influence of energy waves above a determined frequency and to restore said frame to a neutral position on its pivot when it is moved therefrom due to transmission to the vibrating member of energy waves below said frequency.

10. In a reproducer for a sound reproducing mechanism, a pivoted stylus holder, a stylus operated vibrating member, said stylus holder connected with said vibrating member, a frame supporting said vibrating member, a support for said frame, means pivoting the frame to said support on an axle'permanently situated between the axis of the stylus holder and the point of maximum wave generation between the vibrating memher and the support, and the pivot of said frame being on an axis substantially parallel to the axis about which the stylus holder moves.

11. In a reproducer, a support, a frame, a vibrating member carried by said frame, means pivotally connecting the frame to the support, a stylus holder for delivering vibrations to said vibrating member, and magnetic means carried by said support and with which said vibrating member cooperates.

12. In a reproducer, a support, a frame, means pivotally connecting the frame to the support at one side of the center of the frame, a vibrating member carried by the frame, a stylus holder pivoted on said frame and connected with the vibrating member for delivering vibrations thereto, and a magnet carried by the support and with which the vibrating member cooperates.

13. In a reproducer, a support, a frame, means pivotally connecting the frame to the support at one side of the center of the frame, a vibrating member carried by the frame, a stylus holder pivoted on said frame and connected with the vibrating member for delivering vibrations thereto, means balancing the frame on its pivoted connection to yieldingly maintain it in a neutral position, and a magnet carried by the support and with which the vibrating member cooperates.

14,-. In a reproducer, a support, said support including a housing, a frame within said housing, means pivotally mounting the frame within the housing, a vibrating member carried by the frame, a stylus holder pivoted on said frame and connected with the vibrating member for delivering vibrations thereto, and a magnet carried by the support and associated with the vibrating member to cooperate therewith.

15. In a reproducer for sound reproducing mechanism, a pivoted stylus holder, a stylus operated vibrating member, said stylus holder connected with said vibrating member, a frame supporting said vibrating member, a support for said frame, means pivoting the frame to the support on an axis permanently situated between the axis of the stylus holder and the point of maximum wave generation between the vibrating member and the support, the pivot of said frame being on an axis substantially parallel to the axis about which the stylus holder moves, and means holding said frame in a relatively constant but not rigid position relative to the support.

16. In a reproducer, a support including a housing, a pair of pole pieces within the housing and spaced apart and having an air gap between them, a frame, means pivotally mounting the frame in the housing, an armature pivotally mounted on the frame and 8X- tending into the air gap, and means tending to center the armature in the air gap.

17. In a reproducer, a support, a pair of pole pieces carried by said support and spaced apart and having an air gap between them, a frame pivotally mounted on said support and having a neutral position, means tending to hold said frame against movement from its neutral position, an armature pivotally mounted on said frame and extending into said air gap, and means tending to center the armature in the air gap.

18. In a reproducer, a support, a pair of pole pieces carried by said support and spaced apart and having an air gap between them, a frame pivotally mounted on said sup port and having a neutral position, compressible means tending to hold said frame against movement from its neutral position, an armature pivotally mounted on said frame and extending into said air gap, and means tending to center the armature in the air gap.

19. In a reproducer, a support, a pair of pole pieces carried by said support and spaced apart and having an air gap between them, a frame pivotally mounted on said support and having a neutral position, spring means tending to hold said frame against movement from its neutral position, means for varying the tension of said spring means, an armature pivotally mounted on said frame and extending into said air gap, and means tending to center the armature in the air gap.

20. In a reproducer, a support, a pair of pole pieces carried bv said support and spaced apart and having an air gap between them, a frame, means pivotally mounting the frame on the support, the pivoting means for the frame being located at one side of the air gap, an armature, means pivotally mounting the armature on the frame, said armature eX- tending into said air gap, and said pivotal mounting means for the armature being so located that the pivotal mounting for the frame is located between it and the air gap.

21. In a reproducer, a support, a pair of pole pieces carried by said support and spaced apart and having an air gap between them, a frame, means pivotally mounting the frame on the support, said frame having a neutral position, the pivoting means for the frame being located at one side of the air gap, means tending to hold said frame against movement from its neutral position, an armature, means pivotally mounting the armature on the frame, said armature extending into said air gap, means tending to center the armature in the air gap, and said pivotal mounting means for the armature being so located that the pivotal mounting for the frame is located between it and the air gap.

Signed at London, England, this 2nd day of September, 1926.

FRANK ALLEN MITCHELL. 

